1. Field of the Invention
This invention relates to magnetometers and, more particularly, to driven helium magnetometers.
2. Brief Description of the Prior Art
Magnetometers are devices for sensing magnetic fields, generally weak magnetic fields, wherein energy and angular momentum are transferred from a light source to a cell containing an isotope of helium (.sup.3 He or .sup.4 He). The .sup.4 He methodology will be described here with the understanding that the laser source can be applied to the .sup.3 He system as well. The .sup.3 He and .sup.4 He systems differ in that .sup.4 He exhibits an "electron spin resonance" which is monitored as described below. .sup.3 He, on the other hand, exhibits a nuclear magnetic resonance which is monitored via a set of pick-up coils in which an electromotive force has been induced as a result of the atoms precessing in the magnetic field. The .sup.4 He approach to be described places more stringent tolerances upon the laser characteristics, particularly in terms of its amplitude and frequency stability. Otherwise, the laser diode requirements are much the same. The impingement of the light onto the helium isotope, in concert with an electrical discharge, causes the isotope to have magnetic sensitivity, such as sensitivity to changes in the magnetic field of the earth, passing metallic objects, such as submarines, and the like. The helium atoms precess at known frequencies for different magnetic fields. The transparency of the helium cell at 1082.9 nm. with .sup.4 He is a function of the optical resonance of the atoms within the helium cell. A radiation detector positioned on an axis containing the helium cell and across the cell from the light source detects the light when transmitted through the helium cell and produces an electrical signal in response to the intensity of the transmitted light. The optical resonance of the helium cell is modulated by a voltage controlled oscillator (VCO) which provides a variable output frequency to a pair of coils positioned on the sides of the helium cell and electrically connected to the VCO to receive the variable output frequency. The pair of coils produces a variable magnetic field at the precession frequency of the helium metastable isotope that is mathematically related to the magnetic field that is being measured and is thus in resonance with the precession of the atoms within the helium cell. A feedback loop includes a demodulation circuit that is electrically connected between the radiation detector and the VCO and demodulates the electrical signal from the radiation detector to produce a drive voltage for driving the VCO. A frequency counter counts the output signal from the VCO with the magnetic field that the magnetometer is measuring being mathematically related to the measured frequency.
There are two typical radiation sources used for optical pumping of a magnetometer based upon a helium isotope resonance cell, these being helium lamps and lasers. Conventional commercial helium isotope magnetometers use a helium lamp as a source of optical radiation for optically pumping the resonance cell and for optically observing the state of the helium isotope sample in the resonance cell. There are several disadvantages associated with the lamp pumping technique. The lamp radiation intensity is less than optimum for many applications and the lamp radiation contains spectral lines corresponding to the 2.sup.3 S.sub.1 -2.sup.3 P.sub.0, 2.sup.3 S.sub.1 -2.sup.3 P.sub.1 and 2.sup.3 S.sub.1 -2.sup.3 P.sub.2 transitions of .sup.4 He. These spectral lines derive from transitions between the triplet P energy levels and the triplet S energy levels and have center wavelengths of 1082.91 nm, 1083.01 nm and 1083,03 Din, respectively. Radiation of all three transitional wavelengths combined is less effective than radiation of one transitional wavelength.
It has been recognized that a single line laser radiation source could overcome the disadvantages of helium lamp pumping. However, no laser had been recognized as being suitable for this purpose. Prior art attempts in this direction are set forth in the patents of McGregor (U.S. Pat. No. 4,780,672), Schearer et al. (U.S. Pat. No. 4,806,864) and Slocum (U.S. Pat. No. 5,036,278), however none are known to have been commercially successful. The apparent problem with the prior art laser approach is that the issues of stability and control of the source with respect to the optical pumping lines are not addressed. It is imperative that the frequency of the laser light be locked to a particular one of the absorption lines of the helium, ell in order to provide a high sensitivity magnetometer. Of the above noted prior art, McGregor-does not discuss the subject of laser stability or provide a detailed description of the laser source to be used. Schearer et al. proposes a solid state laser solution to allow for high sensitivity operation of a helium magnetometer, however, due to the frequency discrimination in that system, it is optomechanical in nature and has inherent instabilities. Slocum discusses the use of a semiconductor laser for helium optical pumping. However, from a magnetometer standpoint, his use of a temperature-cooled, current controlled, Fabry-Perot laser diode would not allow for a high sensitivity laser pumped magnetometer due predominantly to mode competition and other sources of frequency and amplitude instability inherent in a Fabry-Perot laser diode.